Understanding the higher functions of the human brain is one of the most complex and exciting problems in all of neurobiology. Recently, as neuroscience has adopted new tools from the physical and engineering sciences, striking advances in that understanding have occurred. The present research addresses the problem of the neuroanatomy and neurophysiology of selective attention, those brain mechanisms that allow some stimuli to be processed more rapidly or effectively than others. This problem may be approached by studying the processing negativity, an indication of selective attention that can be routinely measured in the electrically recorded event related potential. The intracranial origins of the processing negativity are presently unknown. However, by mapping the magnetic fields associated with the processing negativity, the intracranial locus of the cellular currents giving rise to the electrically recorded scalp potential may be identified with millimeter accuracy. Further, a new and complementary method for extracting localizing information from measures of scalp electrical activity, current source density analysis, will also be employed. Current source density analysis is theoretically sensitive to radially oriented currents, whereas neuromagnetometry is differentially sensitive to electrical currents within tangentially oriented cells. In a series of five major experiments, the differential contributions of radially and tangentially oriented cells to brain electrical and magnetic fields is first studied. Then, in two experiments, the effects of current intensity on brain electrical and magnetic fields is assessed for both the auditory and the visual system. The final pair of experiments employs well-established behavioral methods to elicit and map the attention-dependent processing negativities of the auditory and visual sensory systems. This program research not only advances the understanding of the neurophysiology of human cognition, but is of potential value in identifying an organic basis in patients with attentional disorders.